Current therapeutic approaches for the repair of the injured lung tissue have had no major long-term benefit in restoring pulmonary function. We have set forth the concept that pulmonary capillary endothelial cells (PCECs) are not just passive conduits that deliver oxygen and nutrients but rather by establishing a supportive niche play a key role in lung regeneration and repair. The overarching goal of this project is to define the mechanism by which after lung injury, activated PCECs through production of growth factors, defined as angiocrine factors, support alveolar regeneration without provoking aberrant fibrosis. We have established the phenotypic definition of PCECs and have shown that after left lung pneumonectomy (PNX), activation of the VEGF-A receptor-2 (VEGFR2) and FGFR1 expressed on the PCECs leads to upregulation of the metalloproteinase MMP14. MMP14 via unmasking cryptic EGF-receptor ligand domains stimulate alveolar regeneration. Notably, transplantation and engraftment of wild-type PCECs expressing MMP14 into the lung of VEGFR2/FGFR1 deficient mice restores lung alveolarization without stimulating fibrosis. However, the mechanism by which PNX activates PCECs to produce angiocrine factors is unknown. Our preliminary data indicate that after PNX, hyperoxia and bleomycin-induced lung injury, myeloid cells and platelets are recruited to the injured lung tissue and by deposition of VEGF-A and stromal derived factor-1 (SDF- 1, CXCl12) activate their cognate receptors VEGFR2 and CXCR4 expressed on PCECs to produce angiocrine factors initiating lung repair. Based on these data, we hypothesize that after lung injury, hematopoietic cells are recruited to pulmonary capillary vessels and by deploying VEGF-A and SDF-1 stimulate VEGFR2+CXCR4+ PCECs to produce alveolar-active angiocrine factors that support lung repair, while preventing aberrant fibrosis. We plan to leverage lung injury models and technologies developed in our lab, including a new approach to reprogram amniotic cells into vascular endothelial cells (rAC-VECs) to execute the following experiments: Aim 1. Dissect the mechanism by which activation of CXCR4 in VEGFR2+ PCECs elicits and maintains angiocrine-mediated lung repair. Aim 2. Examine the role of recruited hematopoietic cells to damaged lung vessels in mediating PCEC activation, angiocrine factor production and lung repair while preventing aberrant fibrosis. Aim 3. Determine the role of reciprocal crosstalk between hematopoietic cells and PCECs in inducing angiocrine signaling and accelerating alveolar regeneration and repair. Our proposed experiments will set the stage for development of pre-clinical strategies in which by proper activation of PCECs or transplantation of lung-specific engineered PCECs will allow for stimulating lung repair, thus improving respiratory functions and minimizing maladaptive remodeling into fibrotic tissues.